Aidan Finn is a Hyper-V advocate so take what he says with a grain of salt, but after a solid comparison he summarizes the status quo as:

"The facts speak for themselves. WS2012 Hyper-V does more, scales out more, is build as the foundation for an enterprise cloud, and is effectively free. You can go ahead and use vSphere 5.1 if you want, but why would you wan to pay more for less?"